Submillimeter-wave heterodyne receivers are important for a number of applications, from providing quantitative molecular abundance profiles in atmospheres to detecting contra-band. The current generation of receivers relies on metal waveguide blocks made using conventional precision machining tools such as end mills. For real time imaging capabilities and for large fields of view it is highly desirable to have two dimensional detector arrays, and therefore novel approaches to building compact waveguide architectures are needed.
CNC metal machining is a highly refined method capable of producing terahertz circuits, but the cost is high due to the serial nature of the process.
Micromachining of submillimeter-wave and terahertz circuits is a very attractive approach for terahertz waveguide components since it offers the potential for lower cost and better precision fabrication. See, for example, V. Lubecke, K. Mizuno, and G. Rebeiz, “Micromachining for terahertz applications,” Microwave Theory and Techniques, IEEE Transactions on, vol. 46, no. 11, pp. 1821-1831, November 1998. Micromachining offers the potential for batch fabrication at photolithographic accuracies, thus reducing the cost per component while improving precision and uniformity. This type of fabrication technology could enable the development of multi-pixel terahertz systems and novel components that are not compatible with CNC metal machining.
Several different micromachining techniques exist for fabrication of terahertz circuits. Thick, permanent resist such as SU-8 is used to build waveguide structures and has attracted attention due to the minimal equipment requirements and the high aspect ratio features it can produce. See, for example, X. Shang, M. Ke, Y. Wang, and M. Lancaster, “Micromachined W-band waveguide and filter with two embedded H-plane bends,” Microwaves, Antennas Propagation, IET, vol. 5, no. 3, pp. 334-339, 21 2011; and C. H. Smith, H. Xu, and N. Barker, “Development of a multi-layer SU-8 process for terahertz frequency waveguide blocks,” Microwave Symposium Digest, 2005 IEEE MTT-S International, pp. 439-442, June 2005.
LIGA is a German acronym for Lithographie, Galvanoformung, Abformung (Lithography, Electroplating, and Molding) that describes a fabrication technology used to create high-aspect-ratio microstructures. See W. Bacher et al., The LIGA technique and its potential for microsystems—a survey, IEEE Trans. Industrial Electronics, 42, 431-441, October 1995. The LIGA technique offers the possibility to manufacture microstructures with arbitrary lateral geometry, lateral dimensions down to below 1 μm and aspect ratios up to 500 from a variety of materials (metals, plastics, and ceramics). LIGA focuses on thick resists similar to SU-8 as molds for electroplating, and thus can be used to build-up metal waveguides. See, for example, J. Stanec and N. Barker, “Fabrication and integration of micromachined submillimeter-wave circuits,” Microwave and Wireless Components Letters, IEEE, vol. 21, no. 8, pp. 409-411, August 2011; C. Nordquist, M. Wanke, A. Rowen, C. Arrington, M. Lee, and A. Grine, “Design, fabrication, and characterization of metal micromachined rectangular waveguides at 3 THz,” in Antennas and Propagation Society International Symposium, 2008. AP-S 2008. IEEE, July 2008, pp. 1-4; and E. Cullens, L. Ranzani, K. Vanhille, E. Grossman, N. Ehsan, and Z. Popovic, “Micro-fabricated 130-180 GHz frequency scanning waveguide arrays,” Antennas and Propagation, IEEE Transactions on, vol. 60, no. 8, pp. 3647-3653, August 2012.
These resist based technique have some disadvantages. SU-8 processes are very challenging to stabilize and the resist is difficult to deposit uniformly, reducing the precision of each layer thickness or requiring an additional processing step such as lapping. LIGA suffers from similar problems, as electroplating a flat layer of tens to hundreds of microns thick is very difficult, so lapping is also usually required to planarize each layer.
Recent studies have been successful in the fabrication of silicon micromachined components but there is still a lack of effective methods to characterize those circuits. In particular, coupling between the micromachined waveguide and standard metal waveguide flanges suffers from misalignment problems due to the difficulty of aligning to non-metal machined waveguide components.
There is a need for improved methods for fabricating and using submillimeter wave and terahertz devices.